ABOUT FOUNDATION DRAINAGE
Run it to daylight
Wet basements are common. Even some new-home owners complain of wet basements.
If the slope of the building site allows, perimeter drains should connect to solid pipe that runs to daylight. The solid pipe should be sloped at a minimum pitch of 1/4 inch per foot, although a steeper slope is better. If there is more than 200 linear feet of foundation, add a second outlet or increase the size of the outlet pipe from 4 inches to 6 inches.
When there isn’t enough pitch on the lot, the exterior drains should be connected to a sump pump in the basement via a 6-inch line that penetrates the footing near the sump location.
MORE ABOUT FOUNDATION DRAINAGE
Some sites are too wet for basements
Water that seeps through a basement foundation or is forced upward by hydrostatic pressure can be collected in an interior drain system and routed to a sump for removal. However, if you know that groundwater is likely to be so big a problem that you have to relieve hydrostatic pressure with a perimeter foundation drain system and sump pump, you should seriously consider something other than a full basement foundation.
INSTALLING AN EXTERIOR DRAIN
Dig a trench as deep as the bottom of the footings.
Lay filter fabric first. Unroll 6-foot-wide filter fabric along the trench, lapping the material up the sidewalls of the foundation. Spread the excess fabric away from the foundation.
Add crushed stone and pipe. Over the filter fabric, lay a 3-inch layer of crushed stone, and then install the 4-inch rigid PVC pipe all the way around the foundation. The perforated pipe can be installed level. Window wells should be tied to the drain with solid 4-inch PVC. Add crushed stone to a level about 8 inches above the top of the footing, and then pull the excess fabric over the top of the stone and lap it against the foundation wall.
Finish with coarse sand. A 6-inch layer of coarse sand spread on top of the fabric will prevent soil from washing into the fabric and clogging its pores.
INSTALLING AN INTERIOR DRAINAGE SYSTEM
Create a sub-slab drainage field.
Put down an 8- to 10-inch-deep layer of crushed stone before the basement floor is poured so that the entire area beneath the slab drains. Above the crushed stone, install a layer of extruded polystyrene insulation topped with a puncture-resistant vapor barrier, such as cross-laminated high-density polyethylene, which will prevent any below-grade moisture from rising into the basement.
Install an plastic interior perimeter drain.
In most cases, this consists of perforated 4-inch pipe. Proprietary drainpipe systems are also available, usually at a higher cost. All are designed to pick up water where the basement wall meets the the floor and drain it to a sump, from which it can be pumped out.
Put in a sump pump. Water that’s collected on the inside of the foundation is piped to a cavity, or sump, set below the level of the floor. Use a pump that’s automatically activated by rising water to move water outside and away from the house. If local codes allow, the sump can be connected to the sewer system. (A sump pump could be connected to exterior footing drains if they run to daylight away from the house.)
Battery-powered backup pump.
In areas where flooded basements are common, a battery backup system for the sump pump ensures that the system will work when the power goes out. A maintenance or inspection schedule for the sump pump should be included in the homeowner’s manual. Installing a sump-pump pit cover that achieves an airtight seal will improve the home’s air tightness and reduce the risk of radon entry.
Worm’s-Eye View
Footing-drain problems are expensive to fix
Building codes require perimeter drains around the outside of basement footings. They are not difficult to install properly before the foundation has been backfilled, but they are costly and disruptive to put in after the fact.
As long as you’re going to protect the bottom of the foundation of a house from water, it’s worth doing right.
See below for:
Key Materials
Image Credits: Krysta Doerfler/Fine Homebuilding #189
Pick the right drain line
The 4-inch black line that comes in coils is cheaper than rigid PVC pipe, but it’s not as crush-resistant, and the narrow slots in pipe walls are more likely to clog than the holes in rigid pipe.
Design Notes
Integrate water-management strategies
The foundation drain shouldn’t do all the work. If groundwater is managed well, underground drains become just one part of a bigger system. Carefully grading the yard can go a long way toward keeping water away. Surface drains and gutters can catch much of the water that does reach the house.
Drainwater can work for you. If the grade allows drains to reach the surface, a rain garden might be a good option. Then water isn’t just diverted, it also provides valuable irrigation to the landscape.
Builder Tips
Image Credits: Dan Morrison
Backfill with quick-draining material
It may be tempting to backfill a basement foundation with excavated soil, but it’s best to place coarse, granular material like crushed stone or bank-run gravel against the foundation to encourage drainage. A cap of soil with a high clay content near the surface will encourage surface water to flow away from the foundation.
The Code
The Code
Sections 404, 405, 406, and 801 of the International Residential Code (IRC) relate to foundations and below-grade habitable space. All code references are to the IRC unless otherwise specified.
Drainage
Concrete & CMU foundations that contain habitable or usable space need drains [405.1], unless there is good natural drainage [405.1X]. Use filter fabric over drain fields [405.1] and at least 2 inches of stone under pipes [405.1]. If the soil is expansive or collapsible, extend gutter downspouts 5 feet from the building or to an approved drainage system [801.3].
Water-proofing, Damp-proofing, & Backfilling
Below-grade basement walls need damp-proofing [406.1], but if the water table is high, use water-proofing instead [406.2]. Parge CMUs before damp-proofing [406.1], and lap and seal all joints in water-proofing [406.2].
Don’t backfill until foundation walls are anchored to the floor framing [404.1.7] (except walls supporting less than 4 feet of unbalanced backfill [404.1.7X]).
Illustration: from Code Check Building 2nd Edition. click to buy .
OTHER CONSIDERATIONS
Keeping your basement dry pays many dividends: a dry basement is less likely to have mold, and the house may have better indoor air quality and fewer moisture problems in the attic. If you value these dividends, invest in high-quality, crush-resistant footing drains of an adequate diameter, and backfill the entire foundation with coarse granular material that drains well.
It’s better to run footing drains to daylight than to depend on a sump pump. While that may require more excavation, it’s money well spent.
DRAWING LIBRARY CONSTRUCTION DETAILS
WHAT’S WRONG WITH THIS PICTURE?
A footing drain the hard way
These workers are excavating to install a footing drain on an older house — an awkward and expensive task. Most older houses on stone foundations were built without footing drains.
GREEN POINTS
LEED-H ID2 (Durability Management Process) has prerequisites and 3 points for third-party certification of durability processes/practices.
NGBS Under Ch. 6, Resource Efficiency: 4 points for well-designed foundation perimeter drainage as part of durability measures (602.3).
74 Comments
Gravel Fill
I have seen the system fail as drawn. Clay soils backfilled above this terrific pipe & gravel system can simply trap water against the wall underground and cause a leak in the foundation wall. Always install clear gravel wrapped in filter fabric at least 1' wide the entire height of the trench, from pipe up to grade. This disallows the miraculous "water running uphill" phenomena that every foundation wall contractor has witnessed, where underground water under hydrostatic pressure acually travels upward and misses the trench drain at the low point (as drawn) , instead building pressure in the soil against the wall and leaking into the basement.
Another problem with exterior foundation footing drains
I completely concur with the above posting by Alan.
Additionally, it is not uncommon to find downspouts feeding into exterior perimeter drain systems.
These downspout connections provide an ideal entry point for leaf and twig debris from the homes' gutters. Once down in the drain line this debris accumulates and eventually turns into soil. I've rarely seen an exterior perimeter drain plumbed with appropriately spaced clean out ports. This may be because the drain is installed by the concrete contractor who has done the foundations. There are conscientious builders who install wire leaf guards in the drop tubes at the top of each downspout. Some even create a break between the downspout and place wire mesh over the lead down to the perimeter drain. These precautions are easily dislodged either by seasonal gutter cleaning or by lawn services.
See the "builder tips" tab
Alan,
GBA agrees with you. Click the "Builder Tips" tab, and you'll read, "It may be tempting to backfill a basement foundation with excavated soil, but it's best to place coarse, granular material like crushed stone or bank-run gravel against the foundation to encourage drainage."
footing drains
Backfilling with crushed stone is simply a way of inviting water into your basement. Any backfill soil above the footing drain, in my opinion, should be as impervious or more impervious than surrounding soil. Runoff should be handled at the surface.
Footing Drainage
I read the second comment. Drainage from the roof to the gutters go into a separte drainage system which is installed around the perimter of the building just approximately 2 to 3 feet. This soild pipe drainage systm is for the exclusive use of draining water that runs off the roof, into the gutters to the drain spouts and into this separate drainage system. Anyone, connecting the drain spouts to the footing/foundation drainage system should be shot (pun intended). By doing it right, no problems develop, period.
Further, want to prevent water and moisture into the basement? Before the contractor pours the concrete footing, install 6 mil plastic sheathing on the ground soils. Once the concrete footing and wall is poured and cured, fold up the 6 mil plastic along the surface of the footing up several feet along the exterior and seal the top. This prevents ground water and moiture from wicking up into the concrete through to the interior (plasti poly acts as a barrier) further, this is continuous underside of the concrete slab where 6 mil poly is a standard installation underside the concrete floor anyway. PS: Doing the following with the poly plastic, will eliminate the need to put a small silly plastic pipe in the concrete footing for so-called drainage from underside of the slab.
foundation drains
Question: what is the durability or useful life expentency of an exterior foundation drain: 20-30 years, 30-40 years ??
Thanks
Jacques
It depends on the tree roots
Jacques,
The two main enemies of footing drains are fine silt and tree roots, so the longevity of the system depends on whether the foundation was backfilled with coarse granular material or ordinary soil; the soil type; the rainfall; and the proximity of trees.
If the system is well designed and installed, I'd say the expected lifespan would be considerably longer than 40 years. Installing clean-outs (risers to grade) helps make it easier to diagnose and sometimes fix problems.
GBA drawing misleading
Thanks Martin. I value your comments. And thanks for posting mine. Yes I see GBA agrees with the "gravel the entire depth of trench" method. But check out your drawing above- it shows mystery backfill above and at grade and the nice gravel backfill is only around the pipe at the bottom. Most builders in my experience are very visual folks and study the drawings and then call the backhoe! A picture is worth a thousand words. I am requesting you guys at GBA either re-draw this misleading drawing or at least label it better to say "only allow max. 6" deep of topsoil above your trench full of clear gravel" or something like that. The drawing doesn't match your top-notch written information. Thanks!
I agree. 12 years later, this drawing is still on the website and I just handed it to my contractor this morning. I was surprised to see that the gravel didn't go up the wall with a clay cap above. I just shrugged and thought that I was wrong and GBA is correct. I'm glad to be straightened out.
Matt,
Which drawing are you objecting to? Is it the one shown in the attachment below?
The drawing shows that the crushed stone surrounding the perforated pipe is separated from the backfill material by a "burrito" of filter fabric. In most types of soil, this approach works well. So I'm not sure that I understand your objection to the drawing.
Perimeter Drainage
I am having a house built and they just laid the pvc around the home and covered it with stove. I dug down a bit to see if everything was connected and there are no 90's on the corners and the main outlet to move the water away from the house is not even connected. Doesnt sound right.
these clay soils could just
these clay soils could just be back filled above the pipe that's connected to the gravel system and can simply hold water against the wall, that is why we are recommending you to have the Humidifier Filters attached next to the wall which cause a leak in the foundation of the wall. This is because of the cooling of the water, which also makes the wall weak, and leaves small pores in the wall which cause water leakage.
foundation drain
What if the holes in the pipe were installed up? I was told that because of the soil and the way it was being installed, the holes should be facing up. Reason was because of the high water and the lack of gravel under the pipe. This was to get the water away from and out of the house. With little gravel area around and under the pipe, there was a better chance of keeping the holes clear. Has anyone heard of this?
Response to Michael
Michael,
1. I'm not sure why anyone would want to install a footing drain without any crushed stone under the pipe. The crushed stone is important.
2. A footing drain will work if the holes face upwards -- it just won't drain quite as well as if the holes were facing downward. If the difference in elevation between upward-facing holes and downward-facing holes is 2 inches, then your pipe with upward-facing holes won't lower the water level quite as much. A pipe with downward-facing holes will drain the water 2 inches lower.
does anyone have pictures of
does anyone have pictures of how the system should look like. It's easy to see an example, but sometimes hard to understand and visualize it.
Response to Junior
Junior,
There is an illustration at the top of this page. What type of picture are you looking for?
How would you do it in
How would you do it in practice -" install clear gravel wrapped in filter fabric at least 1' wide the entire height of the trench'? How would you separate the 1' gravel "wall" that goes up from footing to grade from the rest of the fill?
Response to Andrei
Andrei,
You use filter fabric to separate the crushed stone from the backfill. In practice, this will require careful backfilling in small lifts, and it will require some hand work to place the crushed stone.
basement floor leaks water when there is alot of rain
could i install a series of 3" pvc pipes around the outside of my my house close to the foundation to prevent this . i would put the pipes down about 1' below the basement floor and cut off pipe at ground level to try to relieve the water pressure. of course i would dig out the holes first i am trying not to put a sump pump in the basement. any other ideas
Response to Don
Don,
Here are the steps to follow, in order:
1. Adjust the grade around the outside of your house so the soil slopes away from your foundation.
2. If your house is on a hill, make a swale on the uphill side. The swale should drain away from your house.
3. Install roof gutters connected to conductor pipes. The conductor pipes should drain at least 10 feet away from the foundation.
If your basement is still wet after completing all of these steps, it may be necessary to excavate the exterior of your foundation down to the footings, in order to install footing drains, as you suggest. But correct the grading and address gutters first.
water in cellar
i understand the respose given but since i live in an area that when it rains there is alot of water pressure under my cellar floor, would the 3" pvc pipe inserted in the ground approx. 1' below the cellar floor help me to relieve this pressure thus stopping my floor from cracking
Second response to Don
Don,
Yes, your proposed solution would work, if the details were correctly executed. Assuming you are working from the exterior, though, it won't be cheap.
There is no reason to excavate 1 foot below the basement floor; a few inches would work. Be sure to include plenty of crushed stone around your pipe; wrap the crushed stone in filter fabric; and run the pipe to daylight. Include a tee with a riser to grade for a cleanout.
1. In general, no reason and probably a bad idea as if the drain is near (<12 inches) from footer you are undermining (in every sense) the stability of footer
2. However, in soils where you must go deep to reach stable soil, yoru pipe could-and arguably should- be below the slab)
water in cellar
thank you for your quick response. can i just dig a 6" hole down about 6" below the floor. put some crushed stone in and then put the 3" pvc pipe in. should i go bigger then 3". how far do i bring the pipe above the ground. water will not come out of pipe, but it will relieve the pressure?
Third response to Don
Don,
Your suggestion requires excavating a trench down to the depth of your foundation footing. The trench has to be continued downhill, at a slope of at least 1/4 inch to the foot, until the trench reaches daylight, so that any water accumulating in the trench can drain.
Such footing drains are routinely installed in mountainous states like Vermont. If you live in Kansas, "draining to daylight" is probably impossible.
Closed cell spray foam as insulator and water barrier?
We are building a 1500 sq-ft home in CT on the footprint of a uninhabitable 1740 saltbox which was painstakingly dismantled. The site is on a steep slope with high water table and we will have a sump pump in the basement to assist with removal of water from beneath the slab. My concrete contractor usually applies Tuff-n-dri to her exterior foundation walls. I want to insulate the exterior foundation walls with at least 2" of rigid foam.
A friend suggested having 2-lb closed cell spray foam sprayed over the top of the footings right up to the top of the foundation wall. The footprint is only 24'x27' so the price is comparable to having waterproofing and then adhering rigid foam to the walls.
Any issues come to mind; i.e. durability, waterproof, etc?
Secondly, referring to drawings 1-05002 Concrete basement//insulation out//1" rigid insulation, and 1-06001 Foundation perimeter drain//interior and exterior//with foundation damp proofing
Why does the rigid foam rest on the footing in 1-05002 and not on the footing in 1-06001?
Lots more questions coming as we move forward with our first "green" home.
Thank you
Pete Mariano
Insects, termites, mice anyone ???
Response to Peter Mariano
Peter,
Q. "A friend suggested having 2-lb closed cell spray foam sprayed over the top of the footings right up to the top of the foundation wall. ...Any issues come to mind; i.e. durability, waterproof, etc?"
A. Closed-cell spray foam can certainly be used to insulate the exterior of a foundation wall. Of course, the above-grade portion of the foundation insulation will need to be protected from physical abuse and ultraviolet light; most builders use stucco.
My guess is that opinions differ on whether closed-cell spray foam alone amounts to adequate waterproofing. You may want to consider waterproofing measures separate from your insulation layer.
Q. "Referring to drawings 1-05002 ... and 1-06001, ... Why does the rigid foam rest on the footing in 1-05002 and not on the footing in 1-06001?"
A. The drawings in our "Strategies and Details" section are not all drawings of the same house. There are a great many ways to approach construction, and our details offer many options. There is no reason to expect consistency from detail to detail. Detail 1-05002 shows a basement with walls insulated with exterior rigid foam. Detail 1-06001 shows a basement without exterior insulation. If you choose to follow the latter approach, the basement should be insulated on the interior later in the construction process.
Sand backfill
I'm retrofitting my 1940s house with this style footer drain and had a question about the back fill. I want to be sure I'm understanding this right before I bury the section I've dug up so far. I made a trench about 2ft wide, all the way down to approx. 6in below the footer. Then I placed a sediment screen, a few inches of gravel, a black 4in corrugated, perforated, sleeved pipe, a few more inches of gravel and another sediment screen. Assuming that's correct the next layer is sand then the backfill I dug out to start with? If so is there a particular grade/style of sand I should be looking for and how thick of a layer do I need?
Response to Michael Shinn
Michael,
Most of your backfill should be free-draining material like crushed stone, gravel, or sand. The top 6 to 8 inches or so should be regular soil (or clay-rich soil), graded to slope away from the foundation
The pipe is about 4 ft. deep
The pipe is about 4 ft. deep, so now after all the gravel I have about 3.5 ft. to fill, all this needs to be sand or gravel except 6-8 in.? That seems like a lot, especially if I'm supposed to take this all the way out to the surface.
Response to Michael Shinn
Michael,
It's your choice. Many builders decide to backfill with soil from the site. Only you can decide whether local conditions indicate that your basement is likely to get damp, and only you can decide whether you can afford to bring in some crushed stone or other free-draining material.
The decision depends on your budget, and whether the investment in good drainage (and the higher likelihood that your basement will stay dry) is important to you.
IMO you cannot possibly invest too much to ensure a dry warm basement , well up to and including an entire additional floor to house so you dont need basement for anything but storage and utilities-that’s a pretty high. Bar, and even then when you go to sell, many buyers will bolt at first hint of dampness/model they sell. I suppose you coudl install and run continuously a very large whole house dehumidifier, but that can always be added if needed later; accewssighn foudnation completely and redoing drainage system properly is not cheap (lets say 20 grand oin my aarea for 1500 sq foot basement footprint AND replacing shrubs ($$$)
Word
Response to Martin
Martin,
I may be using my terminology incorrectly. Next to the basement wall, for 1ft out, and the whole length of the wall I plan on putting gravel up to grade. But what my question was out in the backyard where the drain continues out to the surface, away from the house, should this also be sand/gravel to the surface? Also would just creek sand be sufficient or do I need something else?
Response to Michael Shinn
Michael,
Around the perimeter of your foundation, you should install perforated pipe. Once that is done, you dig a trench downhill to daylight. In this trench you install solid pipe, not perforated pipe.
The solid pipe can be backfilled with soil from the excavation. There is no need to install crushed stone or sand above the solid pipe.
Slope for the Perforated Pipe along the Footing
Am I reading this correctly: "The perforated pipe can be installed level." If so, can someone kindly explain how the water collected in the perforated pipe is to drain without gravity helping it to move? I am working on a residential project where the length of the foundation drain is 40'. Is it possible that the foundation drain with perforated pipe laid level with the footing would work even if it connects to a soild pipe with 1/4"/foot slope (for the solid pipe section). Any help, insights would be greatly appreciated. Thank you.
Response to Kaushal Raju
Kaushal Raju,
It's true that the article notes that "The perforated pipe can be installed level." But the article also notes that "The solid pipe should be sloped at a minimum pitch of 1/4 inch per foot, although a steeper slope is better."
If water enters the perforated pipe, it is drained away by the solid pipe. As long as the solid pipe slopes away from the foundation, drainage will occur. The perforated pipe will never have more than an inch of water if you follow the advice in this article. If the perforated pipe is installed below the level of your slab, then your slab will never get wet.
But as Steve aback says, NEVER have a drain taht close to hosue lower than slab-undermines the footer !
Level installation of Perforated Pipe
Thank you Martin Holladay for your prompt response; greatly appreciate it. To elaborate my situation: I am putting a foundation drain along the east side of the house, which happens to collect water that drains from the neighbor and into crawlspace. The length of foundation footing on the east side is 40' and I would be installing perforated pipe along the entire length. I want to connect the 40' perforated pipe to a 15' solid pipe which will carry the water to the city sewer. I plan to have a contractor drill underneath the side walk and install a pipe that would discharge storm water into the street sewer. It would be easy to slope the 15' rigid pipe 1/4 inch/foot to get it out all the way to the street. And I would love for this to work since it won't require a sump pump. (1) However, given that the perforated pipe section is 40' and the rigid pipe section is 15', would the drainage still work by gravity. (2) Some explanation about how the water would drain in this scenario would be greatly appreciated. (3) If we wanted to extend the foundation drain (from the 40' east side) to 30' on the south side, how do connect the two perforated pipes together and still be able to drain the water? (4) Would you need to have a slope in this situation?
Also since I am based in California I have to follow the California Residential Code (CRC),whereby the gravel part of foundation drain has to be at least 1' away from the footing and 6" higher than the footing. (5) Would you recommend the perforated pipe installed right along the footing, whereby the bottom of the perforated pipe is level with the bottom of the footing? With the gravel, I guess I can have it extend the way CRC requires it, right? Thank you.
Response to Kaushal Raju
Kaushal Raju,
Q. "Some explanation about how the water would drain in this scenario would be greatly appreciated."
A. The water enters the perforated pipe through the perforations (holes). Once there is 1/4 inch of water in the bottom of the perforated pipe, the water begins flowing downhill (by gravity) through the solid pipe to the sewer. (By the way, I hope that your municipality allows you do connect a footing drain to the municipal sewer system. Some municipalities don't allow this.)
Q. "If we wanted to extend the foundation drain (from the 40' east side) to 30' on the south side, how do connect the two perforated pipes together and still be able to drain the water?"
A. With a 90 degree elbow.
Q. "Would you need to have a slope in this situation?"
A. No. Dead level will work. However, since you seem extremely resistant to my advice, you are free to slope your perforated pipe if you want. The only reason most excavation contractors don't like to put a slope on a 40-foot or 70-foot perforated pipe is that the trench gets very deep and threatens to undermine your footing.
I guess I'm in somewhat of an emergency, but would the same principles apply to an alaskan slab / monolithic slab on grade? I had a slab poured (20" edges tapered to 6") and after some heavy rains I found that my concrete floor was damp (has a darker, wet color and is damp to the touch) in a 8' or so radius out from the center of the slab (the edges weren't wet). There is a roof over it and nothing else in the house was wet--it has to be coming from under the slab. The concrete contractor laid 6-mil plastic down (in addition to XPS) but no drainage system was installed around the perimeter of the slab and this is a very wet area on a slope. I'm terrified about the irrepairable damage that might occur if this happens during the winter (upstate NY) so I'm ready to borrow a backhoe and dig some drainage trenches.
Based on what I've read above what I'm thinking of doing is:
1) Regrade the surrounding area so that the slab sits at least 6" above grade and the ground slopes down at least another 6" over 10' horizontal feet out from the slab
2) Dig an 18" to 24" deep trench around the perimeter of the slab (trench would be right up against the slab/foam). Now I've read in several places that the trench should be at least 18" deep, but should the bottom of it really be deeper than the depth of the concrete slab edges?
3) Line the trench with heavy (contractor grade) filter fabric, with enough excess running up the foundation wall and down slope to fully "encase" the gravel afterwards
4) Fill the bottom of the trench with 3" of crushed stone
5) Lay down a perforated 4" drainage pipe (rigid) with the holes pointing down and at a slope of at least 1/4" per foot
6) Install risers with plugs so when/if it becomes clogged I can clean out the blockage
7) Connect the perforated pipes to solid pipes where the water exits
8) Fill the trench up with crushed stone, leaving a few inches for sand and 6" for top soil
9) Encase the crushed stone with the excess filter fabric
10) Cover the filter fabric with a few inches of sand
11) Finally cover with 6" of top soil
Does this sound like a good idea? Or do monolithic slab foundations require a different drainage solution? Again, concerned about the depth of the trench being below the depth of the slab. And I'm actually considering a second drainage trench further up the hill as a "first line of defense"--during heavy rains, it almost looks like there's a sheet of water running over the top soil and down the slope.
Claython,
I can't make recommendations without a site visit. But assuming your slab has 6-mil poly under it, and assuming you have eliminated the possibility that the moisture is condensation -- remember, when hot humid air contacts a cold slab, condensation is common -- then the next issue to address is drainage.
If it is condensation, the moisture source is the air -- and nothing else you are suggesting will help.
If the slab is on a slope, it's important to handle any moisture coming from the uphill side. The slab needs to be elevated above the grade on all four sides, and you may need a swale on the uphill side. Drainage is important.
Good luck.
> Line the trench with heavy (contractor grade) filter fabric
In cases where the water source is above and I want to move water without it soaking into the ground, I line with plastic sheet. Gravel all the way to the surface (no sand or top soil) is much more effective against a sheet of surface water - even more so when top soil is frozen.
I would not dig a trench deeper than the foundation that is tight against the foundation - move it back a bit and let plastic sheet (extending above the surface and lapped up the wall) direct water into the trench.
Imagine the plastic forming a small underground roof with a rain gutter.
Good Afternoon,
I am trying to figure out how far away from the footing should the 4" PVC Pipe be Installed?
I use these resources for reference:
a) https://pro.homeadvisor.com/article.show.Foundation-Drainage.13702.html
b) https://basc.pnnl.gov/resource-guides/footing-drain-pipe#quicktabs-guides=0
I excavated the footer and found out that the width of it varies from 10" to 16" along its length of 72ft.
According to the link above, code recommends to install pipe and maintain the following :
1)min 6 below elevation of top of slab.
2) 3" of gravel bed below the pipe.
I drew that in cad -see attached
My footer is about 9" thick.
As you can see from the attached sketch to satisfy both conditions I need to excavate below the bottom of footer about 3" to place enough gravel and stay away from the side of footer at 60 degree influence line to make sure I don't underpin the foundation.
By doing all that it gives me slope of 0.35%. (3" over 72ft) and minimum recommended slope is 1% (8.64" over 72ft). Otherwise I will have to move the pipe higher towards the top of slab elevation.
The resource a) recommends placing the pipe 6" away from the side of footer if pipe even with top of footing and right against the footing if the pipe is at the bottom of footing.
I am looking for clarification on were to locate the pipe in my scenario with varying width of footer. (for where the footing is 10" wide there will be 8" of gravel between pipe and footing while where footing is 16" only 2").
I have seen contractors install the pipe right agains the footer but most references recommend to have 3"-6" of stone between.
Does locating pipe 8" away from footer decreases its water removal efficiency?
Can you please let me know what you think about proposed and if there is a better way to locate the pipe.
Additionally, I am planing to use Dimple mat and the sketch shows how I am planing to wrap its end in filter fabric and place gravel above. I haven't seen any details of it so I came up with this. Do you have any comments to proposed?
Thank you
Dmitry,
Your details looks fine. As long as the area between the pipe and the footing are filled with drain-rock it doesn't matter if the distance varies. What you have shown is a very thorough water management strategy. Much more than most projects get.
Thank you Malcolm,
What do you think regarding having only 3 inches to slope the pipe over 72ft if I need to keep pipe between 6 inch below top of slab and not lower than the bottom of footer? Will there be any issues if I start the top of pipe 4 inches below (bring it 2" higher than shown) the top of slab at my highest foundation depth (coordinate 0 ft) and finish at (coordinate 72ft) with bottom of pipe 1" below bottom of footing (moving 1" lower than shown). This way I will have 6" to play with slope over 72'. That gives me 0.7% still 0.3% less than recommended 1%....
Dmitry,
Your perforated footing drains don't have to slope, in my opinion. They can be dead level, all around the perimeter of your building. The only pipe that needs any slope is the unperforated pipe that conducts the water away from the building to daylight.
Thank you Martin.
What do you think about backfill above the gravel up to the surface?
I read here some want it to be free draining sand and some don’t recommend it saying it would provide free draining path from the surface grade down to the tile. They want backfill that is impermeable. (Original earth) What are your thoughts ?
Dmitry,
Around the perforated pipe, you want a "burrito" -- a sausage filled with crushed stone, wrapped in filter fabric. Adjacent to the foundation and above the burrito, you want (ideally) free-draining backfill -- which in some cases is ordinary soil (if you are building somewhere where the soil is sandy), and in other locations needs to be purchased. (Bank-run gravel will work.) The top 6 or 8 inches should be soil -- ideally soil with a high clay content -- to direct rain away from the foundation and to prevent rain from percolating quickly near your house.
Thank you Martin for your reply.
What about slab on grade - does it require or recommended to install French drain at the footer of stem walls?
Please take a look at the attached sketch. I am trying to figure out where French drain installation is not required.
I have a lot that is sloping down from south to north. I have excavated the East and West sides of the basement walls to install the French drain tile.
1) do I need to install the drain at the North side wall where basement slab is at grade level?
2) Do I need to install drain around slab on grade at South side?
3)How to address drainage at the south side of the basement wall that is not accessible due to garage and sunroom.
Thank you
Dmitry,
Talk to your concrete contractor. Your slab on grade is adjacent to a basement -- so your sketch shows a slab on grade that needs to be placed over backfill. Under those circumstances, you have to think about (a) the order of construction, (b) proper slab support, (c) careful tamping of backfill. You may need frostwalls to support the slab. If necessary, talk to an engineer.
Before this slab is placed, you certainly want footing drains around the entire perimeter of the basement.
I agree with Martin, but generically, sloping any drain (pipe or gravel filled trench) increases its maximum flow rate and the velocity. The former effects capacity and the latter effects the ability to move solids (reducing clogging).
Thank you Jon R.
If you get a chance please take a look at my other questions I addressed to Martin #45.
I would like to know your opinion as well.
Martin, this is an existing condition not a new build.
I am excavating to retrofit the drainage.
Dmitry,
In that case, recommendations would come after a site visit. On the uphill side (the slab side), the best approach is to build a swale, so that the grade slopes away from the building (even on the uphill side). What to do with the drainage paths on each side of the swale depends on site conditions.
Understood. So generally you do not recommend installing French drain at the footing of slab on crade foundations ?
Hi,
A contractor told me I need to drain water through the footing (to allow groundwater under slab to drain).
Do I need to put a sump pump if I don't have water? I just want to have the water exit from outside to the footprint of the house.
Any help would be much appreciated.
Chikitin, it is common practice to do as your contractor suggests, at least in my area. Often there is a second perimeter drain at the interior as well, which can double as a radon mitigation pipe.
It's always a good idea to include a sump pit when building new, as it's much harder to install one later. You don't need to include a sump pump unless you find that you have a water problems. If your foundation drains lead to open air, you will likely not need the sump pump. But it's good insurance to install a pump anyway.
Thank you for the prompt response. May I ask what area you are in? I am in the center of North Carolina, USA.
Also, anything should I know about this kind of drainage system so I make sure he is doing it right.
Again thank you so much.
You're welcome. I'm based in Maine. I suggest searching this site for "foundation drainage." (Google works best: https://www.google.com/search?q=greenbuildingadvisor.com%3A+foundation+drainage.)
Thank you, Michael. I should have guessed it from your name. You are in east as I am.
what's more important: that the perf drainage pipe sits on top of 3" of crushed stone or the top of the pipe is situated at a level lower on the footing?
Fable Farm,
Q. "What's more important: that the perforated drainage pipe sits on top of 3 inches of crushed stone or the top of the pipe is situated at a level lower on the footing?"
A. Ideally, the bottom of the pipe should be situated at a lower level than the bottom of the footing. The existence of 3 inches of crushed stone under the pipe is less important, but still desirable.
We are putting floor drains in our conditioned crawl because of the potential for high groundwater during heavy snow melt every 7 years. (We think we're well above the flow but geotech doesn't know exactly where it will be.) We're also in a high radon area so have an active radon system.
How does one deal with a sump pump in a house that is supposed to be tight, both for performance and radon? I see there are airtight sump pump covers, but the pipe that exits to the drywell from the pump is open, and I believe the bottom of the pump is open. What do we do to keep radon out of our drainage system?
I'm not sure I'm clear on the setup you have now--where is the drywell that the pump dumps into?
The drywell is in the yard.
Great.
The water you dump to the drywell will have some radon it it. That's OK. The radon will come out the water and into the atmosphere and get diluted in the air, just like it would if you haven't built the house.
Or maybe I'm misunderstanding the concern.
I have not yet read all of this thread, but here are a few thoughts. Even a level pipe will have an effective “pitch” equal to the inside diameter of the pipe. In other words, water will flow out of a level pipe rather than “pileup” inside of the pipe. But due to surface tension, water droplets or even a shallow puddle will lay at the bottom of a level pipe without flowing out.
I agree that it is best to not make the pipe trench bottom lower than the bottom of the footing. But this effect of weakening the support of the footing by digging deeper than its bottom depends on how near the footing is to the drain tile. If the drain tile is say 14-18” away from the footing side, I would not worry about the tile trench bottom sloping downward from the bottom of the footing to the bottom of the tile; with the total drop being maybe 2.5 inches max. This assumes the soil under and along the footings is properly compacted and is a proper compatible type of soil such as a sand/clay blend.
I would prefer some drop in the drain tile, but it could be less than the standard ¼” per foot drop. I would not have any of the tile diameter above the slab grade.
I would also not place free draining aggregate between the bottom of the tile and the bottom of the trench. That aggregate would not be able to drain except for percolating into the ground below. In the meantime, it would hold the water and possibly contribute is wetness to the home interior as vapor.
If you are not allowed to drain into the city sewer, the next choice is to daylight the drain tile on your property if the finish grade around the house drops low enough on your property to daylight the drain line. Otherwise, a drywell can be built on your property deep enough to hold water below the elevation of your slab. As a last resort, a sump pump could lift the drain water so it can drain off somehow.
Overall my preference is to have an excellent drain tile system, but to solve the water entry issues entirely at the surface with careful detailing and various membranes, so the drain tile never sees a drop of water under normal circumstances.
Taking it to its ultimate conclusion, my preference is to not have a basement.
We're about to install interior footing drains of 4” perforated pipe with filter fabric sock on 1” bed of 3/8” gravel covered with 1” gravel before insulation, VB, and 4” concrete slab. The pipe will lay 1" away from the footing. Same treatment around the entire 25' x 25' basement. Please see the sketch attached.
We had planned for more gravel around the pipe, but we decided to dig down and get a full 8' ceiling height instead, now we're really close to the bottom of the footing with little room to spare.
Questions....
1) the bottom of the pipe should not be lower than the bottom of the footer, doing so risks undermining the footer
2) it's not ideal, but is 1" of gravel above and below enough?
3) the perforated pipe could be laid dead level, but some pitch up to 1% would be better
4) the perforated pipe should be covered in filter fabric
5) the pipe could be laid directly against the footer or held back a few inches
Your thoughts are much appreciated.
Jim,
1. Yes.
2. You don't need any rock below. 1" above will work.
3. Level is fine. We often lay them with slopes well above 1%.
4. I'm not sure that's necessary for an internal drain surrounded by compacted fill and foam, rather than soil.
5. Lay the pipe several inches away from the footing to ensure you get some rock on that side, rather than leaving a void. On the interior side I'd make sure I had at least 12" of drain-rock.
thanks a bunch Malcom. we can leave more room between the footing and the pipe except for this one tight corner where we’d prefer not to move the plumbing any further away from the footing. on the interior side of the pipe we’re level across the whole basement area, there isn’t a crown in the middle of the basement and the pipe and rock aren’t laying in a ditch that abuts the footing - which I know is the typical practice, but we ran out of room. so from the pipe on one side clear across to the other it’s just 6” of gravel. I suppose the safety factor we have is that we’re installing footing drains, water proofing (and insulating) the exterior as well. thanks again.
Do I still need to install a weeping tile on the inside or outside of my 4'-0" concrete frost wall, when I am designing a slab on grade house?
thehivedsignco,
No. Most codes I am familiar with have language something like "must be installed when required by site conditions."
However, if you have a wet site, you may find it useful to divert ground water away from your foundation - and it's a lot easier to install them now, than later.
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